Catalysts based on highly porous nickel materials are well known. Such materials are part of a family of metal alloy derived products sold by W. R. Grace & Co. Conn. under the trademark “Raney®.” These porous materials, when microscopically viewed, take on a sponge-like appearance having tortuous pore channels throughout the nickel particle. Thus, such metal alloy materials are generically viewed as sponge products. The sponge catalyst product is normally referred to in terms of the metal which constitutes the major component of the sponge product. These high surface area products have been found to have sites for hydrogen activation and, thus, exhibit catalytic activity when used in the preparation of various organic compounds, such as, for example, the hydrogenation of nitro-substituted organics to their corresponding amine compound.
In general, sponge catalysts, such as porous nickel catalysts are formed by first producing a base metal-aluminum (preferred) or base metal-silicon alloy using conventional metallurgical techniques. The formed alloy is ground into a fine powder and classified by passing it through a sieve to provide a material having a desired particle size, which is normally less than 500 microns and, preferably less than 75 microns. Larger particles are recycled for further grinding.
The alloy powder is then treated with a solution of a base to leach out a substantial amount of the aluminum metal or silicon present. The base may be selected from either an inorganic (preferred) or organic compound. For example, in conventional processes an aqueous solution having from about 5 to 50 weight percent concentration of an alkali metal hydroxide (e.g., sodium hydroxide) is employed as the leaching agent. The treatment of the alloy is usually carried out at elevated temperatures of from about 40° C. to 110° C. The alloy powder can be directly added to the alkali solution or it can be formed into an aqueous suspension, which is then contacted with the alkali solution. The aluminum contained in the alloy dissolves to form an alkali metal aluminate (e.g., sodium aluminate) with vigorous evolution of hydrogen. When silicon is in the alloy, the base forms the corresponding alkali metal silicate. The powder and alkali are normally allowed to remain in contact with each other for several hours at elevated temperature (e.g., 40°-110° C.) until the aluminum (or silicon) content is reduced to the desired level. The crude sponge catalyst is separated from the reaction liquor and then conventionally washed with water until the wash water has a slightly alkaline pH value of about 8. The pore volume, pore size and surface area of the leached alloy will depend upon the amount of aluminum (or silicon) in the initial alloy and the degree of leaching.
The metal alloy used to prepare sponge catalysts is generally composed of a major amount of a base metal selected from nickel, cobalt, copper, iron or mixtures thereof, alloyed with aluminum and minor amounts of additional stabilizing or promoter metals. These additional metals typically include metals such as iron, chromium or molybdenum, as deemed appropriate for a particular application. The concentration of a base metal at the surface of the sponge after leaching will generally be limited by the concentration of the metal introduced at the alloying stage. Consequently, the enhancement of concentration of a particularly active base metal, e.g., nickel and/or cobalt, at the surface of a sponge, requires use of major amounts of these metals at the alloying stage.
A typical Raney® cobalt or nickel catalyst has up to about 95% of the primary metal, i.e., nickel and/or cobalt. With the price of nickel currently averaging over $3.00 per lb. and that of cobalt averaging about $9.00 per lb., the cost of using such catalysts can be prohibitively expensive, especially in reactions involving inexpensive organic reactants and products, such as the conversion of dextrose to sorbitol or the conversion of nitrites to amines, wherein the catalyst price may be considered a significant cost component of the overall process.
It is highly desirable to provide sponge-based catalysts having a catalytic activity comparable to conventional nickel- or cobalt-containing sponge catalysts, e.g., a Raney® nickel or Raney® cobalt, which catalysts have a reduced overall content of nickel and/or cobalt, and a high concentration of these metals at the surface of the catalyst.